1. Field of the Invention
The present invention relates generally to a method and arrangement for correctly estimating or equalizing data sequences and more specifically to a method and arrangement which is capable of following rapid changes in channel characteristics with a very small number of data processing steps. The present invention is well suited for use in mobile communications system merely by way of example.
2. Description of the Prior Art
It is known in the art of a radio communications system that a receiver using maximum-likelihood sequence estimation exhibits extremely low error rate, particularly in the case of intersymbol interference. This maximum-likelihood sequence estimation technique is disclosed in an article entitled "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference" by G. D. Forney J, IEEE Transactions on Information Theory Vol. IT-18, No 3, May 1972 (prior art paper 1).
In order to meet the situation wherein channel impulse responses vary with respect to time, an adaptive maximum-likelihood receiver has been proposed and disclosed in an article entitled "Adaptive Maximum-Likelihood Receive for Carrier-Modulated Data-Transmission Systems" by Gottfried Ungerboeck, IEEE Transactions on Communications Vol. Com-22, No. 5. May 5, 1974 (prior art paper 2). This type of arrangement or method utilizes a known training sequence, which is previously sent to a receiver in order to enable initial estimation of channel response of later data transmission. Following this, a decision sequence is generated from a sequence estimator with a predetermined delay using an adaptive algorithm.
However, this arrangement suffers from the drawback that the above mentioned algorithm is unable to follow very rapid changes in channel characteristics.
In order to overcome this drawback, a blind Viterbi equalizer was proposed in Japanese patent application No. 3-128494 and corresponding U.S. Pat. No. 5,325,402 (prior art paper 3). Both of the above mentioned applications have been assigned to the same entity as the present invention.
The known arrangement disclosed in the prior art paper 3, assumes that the transmit signal sequences and the characteristics of a channel are both unknown and then estimates a channel impulse response of each of all the signal sequences expected to be transmitted. Then, branch metrics are calculated using the estimated channel impulse responses to which Viterbi algorithm are applied. The channel impulse responses are estimated by solving a channel equation which is specified by three items: (1) candidates of transmitting signal sequences, (2) channel responses and (3) receive signals. This implies that the optimal solution of a channel impulse response is independently obtained at each time point and thus, the arrangement is able to follow very rapid changes in channel characteristics.
However, the blind Viterbi estimator or equalizer should process a large number of states and, accordingly suffers from the problem in that a hardware arrangement is undesirably rendered complicate.
Therefore, there have been proposed several sequence estimators each of which is able to follow rapid changes in channel characteristics using a smaller number of symbol states through the use of past information of survived signal sequences. Such sequence estimators are disclosed in the following articles entitled:
(a) "An adaptive Maximum-Likelihood Sequence Estimator for Fast Time-Varying Fading Channels" by Hiroshi KUBO, et. al., the 1990 Autumn National Assembly of the Electronic, Information & Telecommunication Society of Japan, paper No. B-283, October 1990 (prior art paper 4);
(b) "Joint Data and Channel Estimation using Fast Blind Trellis Search Techniques", IEEE Blobecom '90 Conference Record, pp. 1659-1963, December 1990 (prior art paper 5);
(c) "Reduced-State Blind Viterbi Equalizers" by Akihisa USHIROKAWA, the 1991 Autumn National Conference of the Electronic, Information & Telecommunication Society of Japan, paper No. SB-4-2, September 1991 (prior art paper 6); and
(d) "Adaptive RLS-MLSE for Digital Mobile Radio" by Kazuhiko FUKAWA, et. al., the 1991 Autumn National Conference of the Electronic, Information & Telecommunication Society of Japan, paper No. SB-4-1, September 1991 (prior art paper 7)
Each of the known sequence estimators disclosed in the prior art papers 4-7, estimates a channel impulse response of each state of trellis diagram. Subsequently, the estimated impulse responses are processed using Viterbi algorithm. However, each of the known estimators of the prior art papers 4-7 is required to calculate the channel impulse responses of all the states of a trellis diagram and then implements branch metric calculation. Thus, the prior art techniques of the prior art papers 4-7, has encountered the problem that a large number of process steps is required.